Trigeminal System

I. OVERVIEW, The trigeminal system provides sensory innervation to the face, oral cavity, and supratentorial dura through general somatic afferent (GSA) fibers. It also innervates the muscles of mastication through special visceral efferent (SVE) fibers.

II. THE TRIGEMINAL GANGLION (semilunar orgasserian) containspseudounipolarganglion cells. It has three divisions:

A. The ophthalmic nerve [cranial nerve (CN) V-l] lies in the wall of the cavernous sinus. It enters the orbit through the superior orbital fissure and innervates the forehead, dorsum of the nose, upper eyelid, orbit (cornea and conjunctiva), and cranial dura. The ophthalmic nerve mediates the afferent limb of the corneal reflex.

B. The maxillary nerve (CN V-2) lies in the wall of the cavernous sinus and innervates the upper lip and cheek, lower eyelid, anterior portion of the temple, oral mucosa of the upper mouth, nose, pharynx, gums, teeth and palate of the upper jaw, and cranial dura. It exits the skull through the foramen rotundum.

C. The mandibular nerve (CN V-3) exits the skull through the foramen ovale. Its sensory (GSA) component innervates the lower lip and chin, posterior portion of the temple, external auditory meatus, and tympanic membrane, external ear, teeth of the lower jaw, oral mucosa of the cheeks and floor of the mouth, anterior two-thirds of the tongue, temporomandibular joint, and cranial dura.

D. The motor (SVE) component of CN V accompanies the mandibular nerve (CN V-3) through the foramen ovale. It innervates the muscles of mastication, mylohyoid, anterior belly of the digastric, and tensores tympani and veli palatini. It innervates the muscles that move the jaw, the lateral and medial pterygoids (Figure 10-1).

III. TRIGEMINOTHALAMIC PATHWAYS (Figure 10-2)

A. The ventral trigeminothalamic tract mediates pain and temperature sensation from the face and oral cavity.

1. First-order neurons are located in the trigeminal (gasserian) ganglion. They give rise to axons that descend in the spinal trigeminal tract and synapse with second-order neurons in the spinal trigeminal nucleus.

2. Second-order neurons are located in the spinal trigeminal nucleus. They give rise to decussating axons that terminate in the contralateral ventral posteromedial (VPM) nucleus of the thalamus.

3. Third-order neurons are located in the VPM nucleus of the thalamus. They pro-

Pain Innervation Ear

Figure 10-1. Function and innervation of the lateral pterygoid muscles (LPMs). The LPM receives its innervation from the motor nucleus ot the trigeminal nerve found in the rostral pons. Bilateral innervation of the LPMs results in protrusion of the tip of the mandible in the midline. The LPMs also open the jaw. Denervation of one LPM results in deviation of the mandible to the ipsilateral or weak side. The trigeminal motor nucleus receives bilateral corticobulbar input. CN — cranial nerve; LMN = lower motor neuron; U,M.V = upper motor neuron.

Figure 10-1. Function and innervation of the lateral pterygoid muscles (LPMs). The LPM receives its innervation from the motor nucleus ot the trigeminal nerve found in the rostral pons. Bilateral innervation of the LPMs results in protrusion of the tip of the mandible in the midline. The LPMs also open the jaw. Denervation of one LPM results in deviation of the mandible to the ipsilateral or weak side. The trigeminal motor nucleus receives bilateral corticobulbar input. CN — cranial nerve; LMN = lower motor neuron; U,M.V = upper motor neuron.

ject through the posterior limb of the internal capsule to the face area of the somatosensory cortex. (Brodmann's areas 3, 1, and 2).

B. The dorsal trigeminothalamic tract mediates tactile discrimination and pressure sensation from the face and oral cavity. It receives input from Meissner s and Pacini's corpuscles.

1. First-order neurons are located in the trigeminal (gasserian) ganglion. They synapse in the principal sensory nucleus of CN V.

2. Second-order neurons are located in the principal sensory nucleus of CN V. They project to the ipsilateral VPM nucleus of the thalamus.

Ventral posteromedial nucleus of thalamus

Face area of postcentral gyrus

Ventral trigeminothalamic tract----

Ventral posteromedial nucleus of thalamus

Face area of postcentral gyrus

Ventral trigeminothalamic tract----

Trigeminal Nucleus Spinal Thalamus

Caudate nucleus

Internal capsule (posterior limb)

- Dorsal trigeminothalamic tract

Mesencephalic nucleus of CN V

Motor nucleus of CN V

Spinal trigeminal nucleus

Spinal cord

Figure 10-2. The ventral (pain and temperature) and dorsal (discriminative touch) trigeminothalamic pathways. CN = cranial nerve.

Caudate nucleus

Internal capsule (posterior limb)

- Dorsal trigeminothalamic tract

Mesencephalic nucleus of CN V

Motor nucleus of CN V

Spinal trigeminal nucleus

Sensory branch of CN V-1 Sensory branch of CN V-2

Sensory branch of CN V-3 Motor branch of CN V-3

* Spinal trigeminal tract

Spinal cord

Figure 10-2. The ventral (pain and temperature) and dorsal (discriminative touch) trigeminothalamic pathways. CN = cranial nerve.

3. Third'order neurons are located in the VPM nucleus of the thalamus. They project through the posterior limb of the internal capsule to the face area of the somatosensory cortex. (Brodmann's areas 3, 1, and 2).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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